Applicants' copending application Ser. No. 235,906, filed Feb. 19, 1981, now U.S. Pat. No. 4,463,405 and assigned to Electric Power Research Institute, discloses a nonfragmenting distribution arrester having an ablative heat shield liner disposed against the interior surface of the porcelain housing. This liner insulates the housing from the thermal shock of an impinging arc precipated by the failure of one or more of the varistor elements within the housing. In addition, the ablative property of the liner material causes the evolvement of a considerable quantity of gas in the presence of an arc, which contributes to a rapid buildup in internal pressure. Before the internal pressure can achieve magnitudes sufficient to burst the housing, one or both of the sealing diaphragms at the ends of the housing and the associated housing end caps are blown off to promote essentially unrestricted and virtually instantaneous internal pressure relief.
While this construction has proven effect in avoiding fragmentation of a distribution arrester housing, its principles are not fully adaptable to the very much larger station class arresters. For one thing, station arresters are typically stacked physically one atop the other and electrically interconnected in series to meet the high voltage ratings of a typical application. Thus, effective internal pressure relief cannot be achieved by having the arrester end structures blow off.
Currently, to inhibit housing fractures under the unusual conditions of internal varistor element failure, station arresters utilize end structures equipped with oppositely directed nozzles for directing the ionized gases generated by an internal fault current arc along the exterior surface of the housing. When these gases meet, an external line-to-ground flashover is created shunting the failing varistor elements. At the high rated fault current withstand of a station arrester, this transfer of the fault current arc from the interior to the exterior of the arrester housing occurs sufficiently rapidly (typically in the first half cycle of a 60 Hz fault current) to save the housing from violent rupture. In order to achieve this flashover, the venting nozzles must of necessity be very restrictive to the free flow of high pressure gases. Thus, if the fault current arc does not rapidly transfer to an external flashover, the internal pressure can rapidly build to a housing fracturing level. Further complicating the matter is the fact that an arrester may fail over a wide range of fault current magnitutes and/or under a variety of adverse weather conditions, e.g., high winds, heavy rain, icing, etc. Possibly, the vent nozzles could become blocked or become even more restricted due to the entry of foreign matter. The effects of these various conditions on the speed of arc transfer is not known. In any case, it is appreciated that should the housing disintegrate, fragments are propelled at extreme velocities, posing serious hazzards to personnel and equipment in the vicinity.
It is accordingly an object of the present invention to provide a high voltage rated lightning arrester capable of avoiding housing fragmentation during a failure mode.
A further object of the invention is to provide a nonfragmenting lightning arrester of the above character which is constructed to reliably inhibit housing failure under all levels of fault current conditions and adverse weather conditions.
Another object of the present invention is to provide a nonfragmenting lightning arrester of the above character which is efficient in construction and reliable in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.